Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum (i.e., calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, and, optionally, other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product.
Calcium sulfate is available in several forms as follows: Dihydrate—CaSO4.2H2O (commonly known as gypsum); Hemihydrate—CaSO4.1/2H2O (commonly known as stucco); and Anhydrite—CaSO4.
When gypsum, (i.e., calcium sulfate dihydrate) is calcined, water is removed from the structure of the calcium sulfate molecule. When one and a half molecules of water are removed from the molecular structure of gypsum, the hemihydrate results, a material used in various compositions in which rehydration occurs during the setting process subsequent to the addition of the water. When two molecules of water are removed from the molecular structure of gypsum, the anhydrite results. Anhydrites formed by calcining at low temperatures are able to rehydrate when exposed to moist conditions. However, if the calcium sulfate is calcined at high temperatures, typically of about 900° F. or more, an insoluble form of calcium sulfate results.
For example, gypsum (CaSO4.2H2O) powder, from sources such as rocks of natural gypsum crushed to make gypsum powder or synthetic gypsum made to be a powder, is heated to a temperature of generally about 250° F.-360° F. The powder converts to hemihydrate which takes the form of CaSO4.1/2H2O. When the hemihydrate is heated to even higher temperatures, the gypsum converts to soluble anhydrite or insoluble anhydrite CaSO4 (dead burn). At high enough temperatures, some of the CaSO4 converts to CaO (quicklime), giving the dead burn a high pH.
Examples of calcium sulfates manufactured by the United States Gypsum Company of Illinois include.
Landplaster: Chemically, landplaster is a dihydrate form of calcium sulfate—CaSO4.2H2O. It is manufactured by grinding gypsum rock to a fine particle size in a roller mill. The median particle size of landplaster is around 30 microns. Landplaster is typically about 80-98 wt % calcium sulfate dihydrate.
Terra Alba: Chemically, Terra Alba also is a dihydrate form of calcium sulfate—CaSO4.2H2O. It is made by fine grinding and air separating a select, white, high-purity gypsum containing about 20% water of crystallization. The median particle size of Terra Alba is around 12 microns.
HYDROCAL (C-Base Gypsum Cement): Chemically, HYDROCAL (C-Base Gypsum Cement) is hemihydrate (alpha) form of calcium sulfate—CaSO4.1/2H2O. The primary use of HYDROCAL cement is in the manufacturing of building products such as joint compound and industrial products such as industrial plasters
SNOW WHITE F&P: Chemically, SNOW WHITE F&P is an insoluble anhydrous form of calcium sulfate. It is manufactured by high temperature calcination of the oversize portion of air classified high-purity landplaster. Only a small specially sized fraction of the high purity landplaster is suitable for producing dead burn. Particles that are too small will agglomerate into amorphous balls and particles that are too big will not calcine completely or will take longer to calcine completely. The calcined material is ground and air separated into a bright white powder. The median particle size of SNOW WHITE F&P is around 8 microns. Its main use is in food and pharmaceutical formulations. The combined water content of SNOW WHITE F&P is less than 0.35%.
CAS-20-4: Chemically, CAS-20-4 is an insoluble anhydrous form of calcium sulfate. It is manufactured by high temperature calcination of a high purity gypsum rock, and, like the Snow White Filler, requires a specially sized fraction of the high purity landplaster. CAS-20-4 is extremely white in color, and its median particle size is around 4 microns. The combined water content of CAS-20-4 is less than 0.20%. The high temperatures required for production of CAS often result in partial conversion of the CaSO4 to lime and as a result the CAS has a high pH.
Another anhydrite (dead burn) calcium sulfate filler available from US Gypsum Co. is CA-5 anhydrous or dead burn calcium sulfate filler which has twice the surface area of SNOW WHITE filler. The ball milling required to produce CA-5 causes the surface of the particles to fracture and the ragged surface attracts adsorbed water. This adsorbed water renders the milled dead burn filler unsuitable for some applications.
There are a wide variety of anhydrite forms. For example, there are soluble and insoluble forms according to their respective abilities to chemically absorb water. Anhydrite III (AIII) is soluble. Anhydrite II is insoluble. The Anhydrite II (AII) forms may be further divided into AII-s gypsum of poor solubility and the inert AII-u gypsum and inert AII-E (which has a high pH). For purposes of this description AII-s, AII-u and AII-E are all dead burn calcium sulfate anhydrites.
The soluble and insoluble anhydrites produced via high temperature atmospheric calcination can only be produced in nonfibrous forms. If the gypsum is calcined via pressure calcination to produce the alpha crystal form, hemihydrate could be produced in fibrous crystalline forms as well as in nonfibrous crystalline forms. After hemihydrate fibers were made in the slurry, the subsequent drying process may have produced some soluble anhydrite when the fibers were dried at a high enough temperature to produce soluble anhydrite. The fibrous materials have been found useful in reinforcement in a variety of solid matrix materials, such as polymeric resins. Due to the high moisture content of the hemihydrate and the instability and tendency of soluble anhydrite to absorb water, the use of these fibrous soluble anhydrite materials has been limited to matrix materials that are not affected by the chemical water in their formation. Where water is present, higher temperature processing (e.g., where the gypsum is used as filler in thermoplastic melts) can release this water and ruin the product. To make a dead burn fibrous filler product, fibrous hemi-hydrate filler was sent to a secondary calcination process where the fibers were gently calcined at temperatures high enough to produce dead burn, yet not break the fibers. This was an extremely costly process.
Dead burn material has many applications including use as filler in thermoplastics, herbicides, foods and pharmaceuticals, cement, plaster additives, etc. For example, dead burn anhydrous calcium sulfate is useful to reinforce thermoplastic, thermoset or plastisol systems. For example, it may be used to reinforce PVC (polyvinyl chloride) piping. Dead burn anhydrous calcium sulfate can also be employed as a titanium oxide extender. The primary markets in which anhydrous calcium sulfate filler products are sold include plastics, specialty cements, caulks and adhesives. The added value of anhydrous calcium sulfate results from its softness, whiteness, acid resistance, and functionality in cement. However, the production of dead burn material is difficult.
Many methods and devices for calcining gypsum are well known. Traditionally, refractories have been used to calcine gypsum in large kettles having a thickened dome-shaped bottom, and the kettle is heated by gas-fired flames in a brick refractory structure. (See U.S. Pat. No. 3,236,509). Other calcining methods and devices have taken the form of refractoryless kettles which use submerged combustion heating systems within the kettle such as disclosed in U.S. Pat. Nos. 4,626,199, 4,629,419 and 4,744,961. One major objective of both refractory and refractoryless kettles for calcining gypsum has been to produce calcium sulfate hemihydrate, better known as stucco, for use in the production of wallboard.
In contrast to such methods for production of hemihydrate or stucco, U.S. Pat. No. 5,954,497 to Cloud et al., incorporated herein by reference in its entirety, discloses a method and system for calcining gypsum to recover calcium sulfate anhydrite. The recovered anhydrite product may be in the form of soluble calcium sulfate anhydrite which is unstable or insoluble calcium sulfate anhydrite which is stable and often referred to as dead burn.
U.S. Pat. No. 5,954,497 discloses feeding gypsum material through two or more stages of calcining to gradually convert the gypsum to dead burn material. In the first stage, the gypsum material contains chemically-combined water which is released by the heating process to self-fluidize the gypsum powder so it will flow through the apparatus. The gypsum powder in the first stage is generally heated to form a hemihydrate product which occurs in a temperature range of about 250° F. to 380° F., or generally less than 400° F. The material is then passed through at least one subsequent stage, preferably two or more stages, so it is heated sufficiently to form calcium sulfate anhydrite. In the subsequent stages, the method includes the steps of heating and simultaneously fluidizing the material with a fluidization media, preferably air, so it will flow through the subsequent stages of the system. The material is then recovered from the process in a form consisting essentially of calcium sulfate anhydrite. The recovered anhydrite product may be soluble or insoluble depending upon the desired use. The insoluble anhydrite is generally referred to as dead burn material. Preferably, the method involves three steps of calcining the gypsum through three calcining kettles to recover calcium sulfate anhydrite from the third kettle. This process requires the specially sized fraction of landplaster to work, “wasting” the majority fraction of the special high purity landplaster reserves.
U.S. Pat. No. 3,650,689 to Cafferata, incorporated herein by reference, produces anhydrous calcium sulfate by autoclaving in saturated steam at a temperature of at least 205° C. However, this is a minimum theoretical temperature and would be commercially impractical since you would need a very long time at this temperature.
There is a need for improved methods of making calcium sulfate anhydrite.
In this specification, percent compositions are weight percents unless otherwise indicated.